The present invention relates generally to anti-drop systems used to prevent free fall of vertically moving doors, such as garage doors or the like. More particularly, the present invention relates to such an anti-drop system having a pawl assembly that, in response to a loss of tension in the counterbalance cable, engages a stop surface adjacent to the door to impede its fall. More specifically, the present invention relates to a pawl assembly biased toward engagement with the stop surface but checked by the force of a taut counterbalance cable attached to the door and lying within the pawl""s rotational plane, whereby release of the tension within the cable, causing the cable to go slack, allows the pawl to swing into engagement with the stop surface and impede the fall of the door.
Devices which prevent the inadvertent free-fall of a vertically movable door, such as a garage door, are known in the art. One type of anti-drop system which has been used in the industry employs a spring-loaded bar that is driven outwardly such that it enters a slot on a rail held adjacent to the door to stop the door from falling. In this system, two horizontally oriented bars housed at the very bottom of the door are mounted side by side. The first bar is rotatable about its own axis and is attached to the lift cable of the counterbalance system of the door by means of a shift pin supported on the first bar. The pin has an oblique slope and is oriented such that when the cable is taut, the pin blocks the axial path of the spring-loaded second rod. When tension is released, however, the pin, due to its oblique slope, shifts to a position that clears the second bar allowing the second bar to move into engagement with the slots formed in the rail supported adjacent to the edge of the door. Since the shift pin is located externally of the edge of the door, clearance must be provided for the shift pin between the door edge and the rail, such that the shift pin is free to clear the second bar. This spacing may allow fluid or debris to gather in the area between the rail and the door. This debris and fluid may gather around the anti-drop mechanism and deleteriously affect its performance by interfering with proper operation of the system or corroding its components.
When operating to stop the door, the spring-loaded bar is driven axially outwardly to project through the slots formed in the rail. In this device, the bar must extend through the slot to effectively stop the door. Partial contact with the slot could cause the bar to deflect from the rail or be deformed such that the bar will not hold the door""s weight. In this device, the end of the second bar is flat lying in a plane parallel to the rail, thereby offering little or no resistance to slow the downward movement of the door. Due to the uninhibited fall of the door prior to stopping, the spring-loaded bar is subjected to a large shock load when it catches the fall of the door. It is believed that this shock load could be sufficient to bend or otherwise distort the bar requiring replacement or repair before the anti-drop mechanism could be reused. In any event, assuming the spring-loaded bar is still functional after stopping the door, the bar must be manually reset and held until tension within the cable is restored sufficiently to retain the bar. Although it has been proposed to incorporate a stop flange in place of the slots formed within a rail, it will be appreciated that, despite this modification, this system has the same disadvantages. In addition, the flange in this system may bend or fail under the shock load created when stopping the door.
As a further disadvantage, when used with spring-type counterbalance systems, the tension on the cable varies dependent on the position of the door. Typically, the greatest spring force and, thus, the greatest tension in the cable, is at the closed position. As the door approaches the open position, the spring tension in the cable is reduced and potentially could be reduced to an extent that the spring force driving the bar is not balanced resulting in inadvertent engagement of the stop mechanism. Moreover, the location of the bar mechanism at the very bottom of the door exposes it to dirt, debris and water that may cause the system to jam or otherwise deteriorate to the point of not performing its anti-drop function.
In another anti-drop system used in the industry, a rotating pawl placed within a housing is attached to the door""s suspension cable. In this system, the rotatable pawl is held within the housing and attached to the cable by an eye that extends outside of the housing. A spring is interposed between the housing and the pawl such that when tension is on the cable, the spring is compressed. When tension is released from the cable, the spring drives the pawl downward where it engages an oblique face of a plunger corresponding to an oblique face of the pawl. This forces the pawl to rotate outward such that a portion of the pawl extends outside of the housing to engage a slot formed in a rail similar to that described with respect to the spring-loaded bar system. To effect the engagement between the pawl and plunger, the housing slides relative to the plunger. When tension is released form the cable, the housing moves downward such that a slot formed in the side of the housing is located at nearly the same height as the plunger. In this way, as the pawl is moved outwardly along the angle of the plunger, its tip can extend through the opening in the housing. The tip is provided with an oblique engagement portion that is turned outwardly to facilitate its extension through the slots in the rail. To permit the tip to rotate sufficiently to engage the slots, the housing must be spaced from the rail, and no provision is made to slow the door prior to impact.
While the use of the pawl reduces the distance that the stopping member must travel to prevent drop of the door and helps to reduce forces that might bend the pawl, this system is subject to the same corrosive elements as the spring-loaded bar system, and, due to its complexity, is even more susceptible to the effects of corrosion, which may cause the system to operate improperly or jam such that repair or replacement is necessary. Also, as in the case of the spring-loaded bar system, the reduction in tension on the cable as the door nears the open position could similarly result in unintended activation of the anti-drop mechanism.
It is therefore an object of the present invention to provide an upwardly-acting door system employing a simple anti-drop system to stop a falling door. Another object of the present invention is to provide an anti-drop system having a rotatable pawl assembly supported on the door, where the presence of a taut counterbalance cable between the pawl and the stop surface controls operation of the anti-drop system. Still another object of the present invention is to provide a cutout on the pawl to at least partially receive the counterbalance cable such that the cable is held within the recess as the pawl rotates toward its engaged position, helping to prevent the cable from interfering with proper engagement of the pawl.
Another object of the present invention is to provide an anti-drop system for an upwardly-acting door employing a rotatable pawl engaging a stop surface adjacent the door to stop the fall of the door, where the anti-drop system slows the downward movement of the door prior to engagement with the stop surface to reduce the shock of stopping the door. A further object of the present invention is to provide a pawl and/or stop surface with a greater frictional coefficient to slow the door prior to the pawl""s contact with the stop surface.
Still another object of the present invention is to provide an anti-drop system that automatically resets upon application of tension to the door cable. A further object of the present invention is to provide a rotatable pawl that is held in a disengaged position by contact with a taut door cable, which, when the cable goes slack, allows the pawl to rotate to an engaged position to stop the door and, upon reapplication of tension to the cable, draws the pawl back into its disengaged position.
Yet another object of the present invention is to provide an upwardly-acting door having an anti-drop system that is less prone to the effects of corrosion or debris. A further object of the present invention is to provide an upwardly-acting door having an anti-drop system constructed of a polymeric material. Another object of the present invention is to provide an upwardly-acting door having an anti-drop system located away from the bottom edge of the door and placed in close relation to a stop surface preventing the entrance of debris or fluid that could corrode or otherwise interfere with the operation of the anti-drop system.
Still another object of the present invention is to provide a method of impeding the free-fall of an overhead door caused by loss of tension in a cable used in counterbalancing the door by interposing the cable between the safety stop assembly and a stop surface such that the cable, when taut, checks the biasing of the safety stop assembly and whereby a loss of tension within the cable releases the biasing force to urge the safety stop into engagement with the stop surface.
In light of at least one of the objects, the present invention contemplates a door system including a door movable between a closed vertical position and an open horizontal position, a cable interconnected to said door near the bottom thereof, the cable extending along a vertical line adjacent the door and being normally, substantially taut, and an anti-drop assembly having a pawl pivotally supported on the door, a stop surface formed adjacent the door and a spring operable to urge the pawl toward engagement with the stop surface, wherein the pawl is oriented such that it rotates in a plane passing through the cable and placed in contact therewith such that the taut cable opposes the biasing force of the spring, whereby upon the cable going slack, the spring biases the pawl into engagement with the stop surface to decelerate the door.
The invention further provides a method of impeding the free-fall of an overhead door caused by loss of tension in a cable used in counterbalancing the door comprising, providing a safety stop assembly adjacent the door adapted to selectively engage a stop surface to impede the free-fall of the door; biasing the safety stop assembly to rotate toward an engaged position with the stop surface; and interposing the cable between the safety stop assembly and the stop surface such that the cable when taut opposes the biasing of the safety stop assembly and whereby a loss of tension within the cable results in biasing of the safety stop assembly toward engagement with the stop surface.